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Ili System Sizing: Certain sites are required to implement flow control for the developed sites. These sites must also control stormwater release rates during construction. Generally, these are sites that discharge stormwater directly, or indirectly, through a conveyance system, into a fresh water. System sizing is dependent on flow control requirements. Sizing Criteria for Batch Treatment Systems for Flow Control Exempt Water Bodies: The total volume of the untreated stormwater storage pond and treatment ponds or tanks must be large enough to treat stormwater that is produced during multiple day storm events. It is recommended that at a minimum the untreated stormwater storage pond be sized to hold 1.5 times the runoff volume of the 10-year, 24-hour storm event. Bypass should be provided around the chemical treatment system to accommodate extreme storm events. Runoff volume shall be calculated using the methods presented in Chapter III-2 - Hydrologic Analysis (p.429). Worst -case land cover conditions (i.e., pro- ducing the most runoff) should be used for analyses (in most cases, this would be the land cover conditions just prior to final landscaping). Primary settling should be encouraged in the untreated stormwater storage pond. A fore - bay with access for maintenance may be beneficial. There are two opposing considerations in sizing the treatment cells. A larger cell is able to treat a larger volume of water each time a batch is processed. However, the larger the cell the longer the time required to empty the cell. A larger cell may also be less effective at flocculation and therefore require a longer settling time. The simplest approach to siz- ing the treatment cell is to multiply the allowable discharge flow rate times the desired drawdown time. A 4-hour drawdown time allows one batch per cell per 8-hour work period, given 1 hour of flocculation followed by two hours of settling. If the discharge is directly to a flow control exempt receiving water listed in Appendix I-E: Flow Control -Exempt Surface Waters (p.133) or to an infiltration system, there is no dis- charge flow limit. Ponds sized for flow control water bodies must at a minimum meet the sizing criteria for flow control exempt waters. Sizing Criteria for Flow -Through Treatment Systems for Flow Control Exempt Water Bodies: When sizing storage ponds or tanks for flow -through systems for flow control exempt water bodies, the treatment system capacity should be a factor. The untreated storm - water storage pond or tank should be sized to hold 1.5 times the runoff volume of the 10- year, 24-hour storm event minus the treatment system flowrate for an 8-hour period. For a chitosan-enhanced sand filtration system, the treatment system flowrate should be 2014 Stormwater Management Manual for Westem Washington 1 1 1 1 1 1 1 1 1 1 L 1 1 1 1 1 1 Volume 11 - Chapter 4 - Page 400 h